Gas sensor and corresponding production method

ABSTRACT

A gas sensor and a method for its manufacture are described. The gas sensor has a solid electrolyte having at least one measuring electrode and one porous protective coating. The measuring electrode has an electrically conductive base layer and a further layer, the further layer god being deposited in the pores of the porous protective coating adjacent to the base layer via galvanic deposition. In order to deposit the further layer via galvanic deposition, the basic body, which has been fused with the base layer and the protective coating via vitrification, is immersed in a galvanizing bath, the base layer being connected as the cathode.

BACKGROUND

German Patent No. 2304464 describes a probe in which a gold or silverelectrode which does not catalyze establishment of equilibrium in thegas mixture and works in conjunction with a platinum electrode that doescatalyze establishment of equilibrium in the measured gas is provided.The catalytically inactive electrode materials cause a competingreaction between the oxygen and the oxidizable and, respectively,reducible gas components to take place at that electrode. Even ifadjustments have been made to ensure high lambda values, very little ofthe free oxygen that is conveyed along with the measured gas reactswith, for example, C₃H₆ or CO; as a result, free oxygen as well as C₃H₆and, respectively, CO reach the three-phase boundary at thecatalytically inactive electrode (non-equilibrium state).

A gas sensor having a measuring electrode and a reference electrodearranged on a solid electrolyte is described in European Patent #466020.In order to create a mixed potential electrode, the measuring electrodeis made of a platinum compound or a ternary alloy that includesplatinum, gold, nickel, copper, rhodium, ruthenium, palladium ortitanium. Herein, the materials may be applied to the solid electrolyteas multiple layers, the alloying step being carried out after thematerials are applied.

U.S. Pat. No. 4,199,425 describes a gas sensor in which a platinumelectrode covered by a porous protective coating is provided. The poresof the protective coating are impregnated with a further catalyticmaterial, rhodium. The rhodium renders the gas sensor sensitive toNO_(x) as well as oxygen. Herein, the rhodium coats the walls of thepores of the entire protective coating; as a result it is impossible tospecify the thickness of the layer in the porous protective coating.

SUMMARY OF THE INVENTION

The gas sensor according to the present invention having thecharacterizing features set forth in claim 1 has the followingadvantage: a sintered sensor element basic body can be used, the furtherlayer being integrated via just one additional deposition step followingthe sintering. As a result, the outer electrode of the sensor elementbasic body can be modified following the sintering. The sensor elementof a Nernst-type lambda sensor, for example, can be used as the sensorelement basic body, it being possible to transform the outer electrodeinto a mixed potential electrode by making certain modifications.Furthermore, it is advantageous that materials that would not withstandthe high temperature at which the sintering is carried out can be usedas the further layers. A further advantage is that the further layersystem, which is directly adjacent to the electrically conductive baselayer, does not completely fill the pores of the porous protectivecoating. As a result, the porous protective coating continues to provideeffective protection, and sufficient gas can access the three-phaseboundary. Herein, the material used as the further layer may be used tomodify the functional characteristics of the electrode of the gas sensorin a specific manner. Herein, this modification may define the specificgas selectivity of the sensor and/or its position within the controlsystem.

Advantageous further refinements of the gas sensor according to thepresent invention and the method according to the present invention canbe achieved via the measures set forth in the subordinate claims. Aparticularly advantageous sensor designed for mixed potentials can beachieved if the layer system is subjected to a thermal additionaltreatment following deposition of the further layer. For example, in thecase of a Pt/Au electrode a temperature range of 1200° C.±100° C. isfavorable. At this temperature, the metal atoms of the further layerdiffuse into the metal of the adjacent base layer. A further advantageis that a cermet layer is used as the electrically conductive base layerwhich, thanks to its ceramic component, creates a solid join with thesolid electrolyte when the ceramic body is sintered. Furthermore, bycreating a plurality of further layers and choosing the layer materialappropriately one can specify the selectivity and also modify thecatalytic activity of the electrode with even greater precision.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a section through a gas sensor according to the presentinvention.

FIG. 2 shows an enlarged sectional view of a first exemplary embodimentof an electrode of the gas sensor according to the present invention.

FIG. 3 shows an enlarged sectional view of a second exemplary embodimentof an electrode of the gas sensor according to the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a gas sensor having a sensor element basic body 10 whosestructure corresponds to that of a Nernst-type oxygen sensor (lambdasensor). Basic body 10 includes, for example, a plurality of ceramicsolid electrolyte foils 11, 12, 13, which are made of, for example,Y₂O₃-stabilized ZrO₂. An outer measuring electrode 15, which is coveredby a porous protective coating 16, is arranged on the outer largesurface of first foil 11. Protective coating 16 is made of, for example,porous ZrO₂ or Al₂O₃. A reference channel 17 is provided in second foil12 and is connected to a reference atmosphere, e.g., air. A referenceelectrode 18, which is arranged on first foil 11 and faces measuringelectrode 15, is arranged in reference channel 17. A heating device 22is integrated into basic body 10, and on third foil 13 electricalinsulating layers 21 are provided, in which heating device 22 isembedded. Heating device 22 is an electrical-resistor-type heatingelement.

According to a first exemplary embodiment, the layer structure ofmeasuring electrode 15 is as shown in FIG. 2. According to thisstructure, an electrically conductive base layer 25, which is made of,for example, a Pt cermet, is arranged on foil 11 of basic body 10.Protective coating 16 is provided on base layer 25. According to FIG. 2,further layer 27 is formed in the pores of protective coating 16 and isadjacent to and on top of base layer 25. This layer 27 is directly incontact with base layer 25. Base layer 25 and further layer 27 formmeasuring electrode 15. It will be discussed how layer 27 ismanufactured below.

Herein, layer 27 may be made of a material that inhibits, i.e., impedes,establishment of equilibrium in the gas mixture on the surface of theelectrode. Such materials include, for example, precious metals (gold,rhodium, iridium), semiprecious metals (palladium, silver), base metals(copper, bismuth, nickel, chrome) or a mixture of these metals. In thepresent exemplary embodiment according to FIG. 2, further layer 27 ismade of gold. As a result, measuring electrode 15 of the sensor shown inFIG. 1 is transformed into a mixed potential electrode that is selectivewith respect to hydrocarbons (HC).

Mixed potential electrodes are electrodes that cannot or can onlyincompletely catalyze the establishment of equilibrium in the gasmixture. Herein, measuring electrode 15, along with reference electrode18, which is made of, for example, Pt and is arranged in referencechannel 17, form a mixed potential sensor. The material of layer 27 ofmeasuring electrode 15, which does not or only incompletely catalyzesestablishment of equilibrium in the gas mixture, causes a competingreaction between the oxygen and the oxidizable gas components to occurat measuring electrode 15. Accordingly, very little of the CO conveyedalong with the measured gas reacts with the free oxygen to form CO₂. Asa result, free oxygen as well as CO reach the three-phase boundary ofmeasuring electrode 15 and contribute to the signal generated there. Apotential difference arises between measuring electrode 15 and referenceelectrode 18, where constant oxygen partial pressure is present thanksto the reference air, and can be detected as an electromotive force by ameasuring instrument 30. The electromotive force is therefore dependenton the oxidizable gas components. Thus one can specify the selectivityof measuring electrode 15 to a given gas type by choosing the materialused for further layer 27 appropriately, so that it is possible tominimize the extent to which it is cross-sensitive to other gascomponents. Furthermore, one can, for example, improve the behavior ofan oxygen sensor at low temperatures by using an Rh layer on a Ptelectrode.

A second exemplary embodiment of a layer system for measuring electrode15 is shown in FIG. 3. On top of base layer 25, layer 27 is created inthe pores of protective coating 16, and on top of layer 27 a secondlayer 28 is created, and on top of layer 28 a third layer 29 is created.In the exemplary embodiment shown in FIG. 3, layer 27 is made of, forexample, gold; layer 28 is made of, for example, rhodium or iridium; andlayer 29 is made of nickel or chrome. This exemplary embodiment showsthat it is easy to achieve a complex, multi-layer electrode structure.By using the layer structure shown in FIG. 3 and/or choosing anappropriate material for layers 27, 28, 29, one can modify, for example,the catalytic characteristics of the electrode in a specific manner.

To manufacture the sensor according to FIG. 1, one uses, for example,the sensor element basic body 10 described. The appropriate functionallayers are applied to foils 11, 12, 13, these being in their green(unsintered) state. Herein, a Pt-cermet paste is applied to the largesurface of first foil 11 to create base layer 25, and a Pt-cermet pasteis also applied to its other large surface to create reference electrode18. Protective coating 16 is, for example, screen-printed or painted ontop of the Pt-cermet paste of base layer 25 on the large surface of foil11. Herein, the material of protective coating 16 contains pore-formerswhich vaporize and, respectively, combust during the subsequent thesintering process so as to form pores. Insulating layers 21 are appliedto foil 13 via screen-printing steps, and heating device 22 is arrangedbetween insulating layers 21. Foils 11 and 13, to which the functionallayers have been applied, are laminated with foil 12, into whichreference channel 17 has first been punched, and sintered at atemperature of, for example, 1400° C.

Following the sintering, basic body 10 is present, its structurematching that of a sensor element of an oxygen sensor for determiningthe lambda value in gas mixtures. In the case of the present exemplaryembodiments, layer 27 according to FIG. 2 or a plurality of layers 27,28, 29 according to FIG. 3 are applied to basic body 10, which is in itssintered state, layer 27 and, respectively, layers 27, 28, 29 beingformed in layer levels in the pores of porous protective coating 16.

Layers 27, 28, 29 are manufactured via galvanic deposition. Toaccomplish this, the ceramic body is placed in a galvanizing bath. Baselayer 25 is electrically connected as the cathode, the connectioncontact point of base layer 13, which is present on sensor element basicbody 10, being used as the contact point. As the anode, a metal, forexample, is immersed in the galvanizing bath, this metal correspondingto the metal of respective layer 27, 28, 29 to be deposited (galvanizingmethod using a sacrificial anode). For example, water-soluble, ionicsalts of the metal in question, e.g., HAuCl₄, IrCl₃×H₂O or RhCl₃×H₂O,are used as the electrolyte.

In order to manufacture a sensor for measuring hydrocarbons, a layersystem according to FIG. 2 is selected, further layer 27 in the form ofa gold layer being deposited on base layer 25, which is made ofPt-cermet, via galvanic deposition. To accomplish this, sintered basicbody 10 is, for example, placed in a galvanizing bath containing anHAuCl₄ electrolyte, a gold anode being used. If a current of 0.5 to 2 mAis applied for 15-50 minutes, gold layer 27 having a thickness of, forexample, 1-5 μm, is deposited on Pt-cermet base layer 25. Herein, layer27 forms in the pores of protective coating 16. After layer 27 has beendeposited, the ceramic body is subjected to an annealing process at atemperature of, for example, 1200° C. During the annealing process, analloy of the Pt of base layer 25 and the gold of layer 27, namely aplatinum-rich gold phase and a gold-rich platinum phase, is formed. As aresult, the catalytic activity of the Pt of Pt-cermet base layer 25 ismodified, and a mixed potential electrode is created as measuringelectrode 15, this being selective with respect to hydrocarbons.

The layer system according to FIG. 3 is also manufactured via galvanicdeposition, the appropriate anode materials and/or the appropriategalvanizing baths being used in sequence during galvanic deposition. Inaddition to the layer systems shown in FIGS. 2 and 3 and describedabove, further combinations and layer systems for electrodes of gassensors are conceivable, these being deposited as a porous layer on anelectrically conductive base layer.

What is claimed is:
 1. A gas sensor comprising: a solid electrolyte; atleast one measuring electrode situated on the solid electrolyte; and aporous protective coating having pores, the protective porous coatingbeing situated on top of the at least one measuring electrode, whereinthe at least one measuring electrode includes an electrically conductivebase layer and at least one further layer, the at least one furtherlayer being formed in the pores of the porous protective coatingadjacent to the electrically conductive base layer, wherein, in the atleast one further layer, the pores of the porous protective coating arefilled with a material, and wherein the at least one further layer has athickness smaller than a thickness of the porous protective coating. 2.The gas sensor according to claim 1, wherein the material of the atleast one further layer modifies functional characteristics of theelectrically conductive base layer by forming an alloy with a materialof the electrically conductive base layer.
 3. The gas sensor accordingto claim 1, wherein the material of the at least one further layer iscomposed of at least one of: precious metals, semiprecious metals, andbase metals.
 4. The gas sensor according to claim 1, wherein the atleast one further layer includes a plurality of further layers appliedto the electrically conductive base layer.
 5. The gas sensor accordingto claim 1, wherein the electrically conductive base layer includes acermet layer.
 6. The gas sensor according to claim 1, wherein theelectrically conductive base layer includes a Pt-cermet layer.
 7. Amethod for manufacturing a gas sensor, comprising the steps of:arranging an electrically conductive base layer on a solid electrolyte;arranging a porous protective coating over the base layer; sintering thesolid electrolyte, the electrically conductive base layer and the porousprotective coating to form a ceramic basic body; and after the sinteringstep, depositing at least one further layer on the electricallyconductive base layer through the porous protective coating via galvanicdeposition such that the pores of the porous protective coating in theat least one further layer are filled with a material, and such that theat least one further layer is adjacent to the electrically conductivebase layer and has a thickness smaller than a thickness of the porousprotective coating.
 8. The method according to claim 7, wherein the atleast one further layer is deposited via cathodic deposition.
 9. Themethod according to claim 7, further comprising the steps of: placingthe ceramic basic body in a galvanizing bath; connecting theelectrically conductive base layer as a cathode using connection contactpoints present on the ceramic basic body; and using as an anode a metalthat corresponds to the material of the least one further layer.
 10. Themethod according to claim 7, further comprising the step of, after thedepositing step, subjecting the electrically conductive base layer, theporous protective coating and the at least one further layer to afurther heat treatment.
 11. The method according to claim 10, wherein atemperature reached during the heat treatment is lower than atemperature reached when the ceramic basic body is sintered.